(moving the discussion here from GH [1]) Apparently there is something weird going on with the device stats ioctls. I cannot get them to work as regular user, while they work for David. A friend confirms the same issue on his system - no access as non-root.
So I made a new empty fs, mounted it, built btrfs-progs-4.17.1 with debug symbols and stepped into search_chunk_tree_for_fs_info(). Everything is fine, all args are correct, right until: (gdb) s 1614 ret = ioctl(fd, BTRFS_IOC_TREE_SEARCH, &search_args); (gdb) s 1615 if (ret < 0) (gdb) p ret $4 = -1 (gdb) p search_args $5 = {key = {tree_id = 3, min_objectid = 1, max_objectid = 1, min_offset = 1, max_offset = 18446744073709551615, min_transid = 0, max_transid = 18446744073709551615, min_type = 216, max_type = 216, nr_items = 30, unused = 0, unused1 = 0, unused2 = 0, unused3 = 0, unused4 = 0}, buf = '\000' <repeats 3991 times>} Looking at the kernel side of things in fs/btrfs/ioctl.c I see both BTRFS_IOC_TREE_SEARCH[_V2} unconditionally require CAP_SYS_ADMIN. So why can Dave get his dev stats as unprivileged user? Does this work for anybody else? And why? :) cheers Holger [1] https://github.com/prometheus/node_exporter/issues/1100#issuecomment-427823190